1. Field of the Invention
The present invention relates to a circuit and method for extracting preselected information from a signal. More particularly, this invention pertains to such method and circuit for extracting the Doppler frequency and phase from a pseudo-noise modulated carrier without a priori knowledge of the modulating code.
2. Description of the Prior Art
The use of a pseudo random bit sequence to modulate a free space signal has acquired significance in the communication arts with many important applications, both military and civilian, either in effect or foreseen. By utilizng such a sequence, often referred to as a pseudo-noise code (PN-code), one may create a modulated sinusoidal carrier of the type known as a direct sequence spread spectrum signal. In such a signal, more of the bandwidth is occupied than is required for transmission. As such, one may attain a number of advantageous characteristics including (1) hiding of signal (2) inherent anti-jam performance resulting from the spreading of signal power (3) transmission of ranging information, (4) lessened sensitivity to signal path anomalies and (5) code division multiple access (CDMA) allowing the transmission of many signals in the same frequency band without interference.
The above-described characteristics of direct sequence spread spectrum signal transmission have proven advantageous in battlefield environments, oil exploration and satellite-aided radio navigation. A prominent application is found in the currently contemplated Global Positioning System ("GPS"), a world-wide network of earth satellite/transmitters that will allow users to fix or determine their locations with respect to known satellite locations. Each of such satellites will transmit a carrier modulated with an identifying pseudo-noise sequence assigned to the satellite. User location with respect to each satellite is determined from Doppler phase and frequency shift information and actual location then determined through survey methods such as triangulation.
In the prior art, the extraction of the Doppler information has required the identification of the modulating pseudo-noise code (PN-code). This greatly complicates the detection process. For example, many direct sequence spread spectrum systems take advantage of the low cross correlation between PN-codes to utilize simultaneous operation of many transmitters at the same carrier frequency, the CDMA technique. This has required, in actual implementation, multiple receiver channels in addition to knowledge of the PN-codes.